The presence of one or more copies of von Willebrand factor type A domain identifies a superfamily of proteins usually involved in biological processes controlled by specific molecular interactions, often adhesive in nature. We have solved the crystal structure of the prototypic von Willebrand factor A1 domain, essential for the antihemorrhagic activity of platelets, in complex with the function blocking antibody, NMC-4, at 2.2 E resolution. This has led to the recognition of a putative binding groove for the platelet receptor, glycoprotein Iba, formed by two adjacent ?-helices and a _-strand. The structure also shows a contact interface between A1 domain pairs, suggesting a hypothetical mechanism for the regulation of protein assembly and heterologous ligand binding mediated by homophilic interactions of type A domains. The binding of the A1 domain to the glycoprotein Iba is essential to arrest bleeding. Certain mutations on the A1 domain abolish bleeding. Certain other mutations increase the affinity of binding. Studying the structures of these mutants and the structure of the modulator botrocetin will be the focus of this proposal.